The present invention relates to a cleaning method in an electrophotographic copying machine for removing residual toner from a drum surface.
The present invention relates, more particularly, to a cleaning system employing a combined developing-cleaning magnetic brush unit.
In an electrophotographic copying machine employing a photosensitive drum around which a charging unit, an exposure unit, a developing unit and a transcription unit are disposed, a cleaning unit is inevitably required to remove residual toner from the drum surface before conducting the next copying operation.
An electrophotographic copying machine has been proposed, wherein the photosensitive drum is rotated more than one complete revolution to produce one sheet of copy. In such an electrophotographic copying machine, a combined developing-cleaning unit can be employed, which performs the developing operation when the photosensitive drum rotates around the first time, and performs the cleaning operation when the photosensitive drum rotates around the second time.
A typical system of the above-mentioned electrophotographic copying machine is described in U. S. Pat. No. 3,647,293 entitled "COPYING SYSTEM FEATURING COMBINED DEVELOPING-CLEANING STATION ALTERNATELY ACTIVATED" issued on Mar. 7, 1972. In this system, a magnetic brush element performs the developing operation during the first revolution of the photosensitive drum. At the second revolution of the photosensitive drum, a bias voltage of the same polarity as the residual charge on the photosensitive drum surface is applied to the magnetic brush element for electrostatically attracting the residual toner from the photosensitive drum surface.
The present inventors have discovered that inversely charged toner is created while the copying operation is repeatedly conducted due to the deterioration of the developer mixture. The inversely charged toner can not be removed from the drum surface because the inversely charged toner has the same polarity as the bias voltage applied to the magnetic brush element during the cleaning step. If the inversely charged toner remains and accumulates on the photosensitive drum surface, a toner layer can be formed on the drum surface (which will be referred to hereafter as "toner filming layer"). The formed toner filming layer changes the physical and chemical characteristics of the drum, and affects the quality of the copy being produced.
Accordingly, an object of the present invention is to provide a novel cleaning method in an electrophotographic copying machine which employs a combined developing-cleaning magnetic brush element.
Another object of the present invention is to provide a cleaning method for effectively removing residual toner from a photosensitive drum surface in an electrophotographic copying machine wherein the photosensitive drum rotates more than one revolution to complete one sheet of copying operation.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a combined developing-cleaning magnetic brush element is disposed in an electrophotographic copying machine, which contacts the surface of the photosensitive drum. During the first revolution of the photosensitive drum, a bias voltage of, for example, -200 V is applied to the magnetic brush element to develop the latent image formed on the photosensitive drum surface. During the second revolution of the photosensitive drum, a bias voltage of, for example, +30 V or no bias voltage is applied to the magnetic brush element to remove the inversely charged toner from the photosensitive drum surface. During the third revolution of the photosensitive drum, a bias voltage of, for example, -200 V is applied to the magnetic brush element to attract the remaining residual toner from the photosensitive drum surface.
In a preferred form, when multiple sheets of copy are desired to be produced from one original sheet, the above-mentioned positive or no bias voltage step is omitted before the last sheet copy. When the transcription operation for the last copy sheet is completed, the positive bias voltage or no bias voltage is applied to the magnetic brush element to remove the inversely charged toner from the photosensitive drum surface. Then, the negative bias voltage is applied to the magnetic brush element for cleaning the residual toner.